This invention relates to protective networks for data transmission systems and more particularly to such a protective network which prevents the transmission of data signals between a data device and a data transmission circuit when the magnitude of the data signals leaving the data device exceed a pre-selected level.
Data transmission systems are well known in the art. Frequently, such systems take the form of a data device such as a computer transmitting and receiving data over a data transmission circuit such as, for example, telephone lines. Due to requirements imposed by the operators of the telephone lines, such systems frequently include protective networks coupling the computers with the telephone lines, the purpose of such protective networks being to prevent damage to the telephone lines by excessively large signals leaving the computer. Conventional protective networks are, in many ways, less than completely satisfactory. Frequently, such protective networks include attenuating circuits which act to lessen the magnitude of the computer emanated data signals reaching the telephone lines when such signals exceed a predetermined level. It will be understood, however, that the magnitude of the data signals leaving the computer is usually adjustable and, therefore, when the magnitude of the data signals leaving the computer exceeds a desired level it is an indication of a malfunction or error in the system. It is therefore obvious that merely attenuating the data signals emitted by the computer and permitting the continuation of transmission may cause unanticipated difficulties. A further difficulty arises from the fact that these systems are usually bi-directional, that is, data signals are being carried from the computer to the telephone lines and simultaneously, data signals are being carried by the telephone lines to the computer. The aforementioned protective networks customarily respond to the total signal magnitude in the transmission path and this includes the sum of the data signals leaving the data device and the data signals entering the data device. It is thus seen that the conventional attenuating protective network may act to attenuate the data signals emitted by the computer even when such signals have a magnitude well below their maximum allowable level in the event that the data signals being transmitted to the computer over the telephone lines (including noise) exceed an anticipated level.